The present invention relates to a plasma light source.
High Frequency (HF) Plasma is a term often applied to mean both Radio Frequency, RF 1-300 MHz) and Microwave 0.3-300 GHz) excited plasmas. Most HF Plasmas used as light sources are fully localised inside the HF field applicator, that is the discharges are sustained in capacitive or inductive circuits and in resonant cavities, coaxial lines and waveguides.
A drawback of an air filled resonant cavity device is that the size of the cavity is determined by the frequency of operation. Technically successful cavity systems have been designed for operation at 2.4 GHz. At suitable frequencies (ISM Industrial, Scientific and Medical-bands) below this frequency the size of the cavity and the associated waveguides is liable to become physically too large for use in commercial lighting systems. It also becomes difficult to design high pressure plasma chambers for such cavities which operate plasmas at combinations of high radiation efficiency and usefully low power, i.e. less than 400 watts, required for most commercial applications. Indeed even at 2.45 GHz obtaining system powers of less than 400 watts with plasmas of the required radiation efficiency can be difficult.
In order to provide plasmas with a high radiation efficiency and operation at powers less than 400 watts it is known to operate plasma chambers within a dielectric filled resonant cavity. While this latter configuration is suitable as a light source for applications such as projection where small source size is the primary benefit being sought, the first configurations had serious limitations for general lighting situations because of the obstruction of a high percentage of light from the source by the opaque dielectric structure. In this configuration less than 50% of the surface area of a bulb is able to emit light into a limited solid angle, 27π steradian, of free space. This surface area is usually maximised by designing a portion of the bulb volume to be external to the cavity.
As shown in our International Application No PCT/GB2008/003829, we have overcome this drawback. In that application, we describe a light source to be powered by microwave energy, the source having:                a body having a sealed void therein,        a microwave-enclosing Faraday cage surrounding the body,        the body within the Faraday cage being a resonant waveguide,        a fill in the void of material excitable by microwave energy to form a light emitting plasma therein, and        an antenna arranged within the body for transmitting plasma-inducing, microwave energy to the fill, the antenna having:                    a connection extending outside the body for coupling to a source of microwave energy;                        
wherein:                the body is a solid plasma crucible of material which is lucent for exit of light therefrom, and        the Faraday cage is at least partially light transmitting for light exit from the plasma crucible,        
the arrangement being such that light from a plasma in the void can pass through the plasma crucible and radiate from it via the cage.
As used in that application:                “lucent” means that the material, of which an item described as lucent is comprised, is transparent or translucent;        “plasma crucible” means a closed body enclosing a plasma, the latter being in the void when the void's fill is excited by microwave energy from the antenna;        “Faraday cage” means an electrically conductive enclosure of electromagnetic radiation, which is at least substantially impermeable to electromagnetic waves at the operating, i.e. microwave, frequencies.        
In this application we use “Faraday cage” in analogous manner, but not restricted to enclosing microwaves but extended to enclosing the electromagnetic waves at the operating frequency whatever that may be in the HF band as defined above. We do not use the term “plasma crucible” in this application.
Plasmas can be created by travelling waves in waveguides and slow wave structures, so called Travelling Wave Discharges (TWD). For lighting purposes one member of this class of discharges, the Surface Wave Discharge (SWD), has in the past been widely assessed as being particularly promising; this is the propagative Surface Wave Discharge SWD. This type of discharge is well known in the literature, electromagnetic energy forms the plasma and the plasma itself is the structure along which the wave is propagated. A practical field applicator for a SWD is a surfatron. Surfatrons are wide band structures that may be used over a frequency range of 200 MHz to 2.45 GHz and have the property that very high energy coupling efficiencies can be achieved. Greater than 90% of the HF energy can be coupled into the plasma. Although SWD's launched by surfatrons have been proposed for lighting applications, these have been aimed at low pressure discharges. The major application for SWD's is large volume sub-atmospheric to atmospheric pressure plasmas for various processes in microcircuit fabrication. For high pressure lighting applications there is a drawback. The volume of the plasma is very dependant on the plasma pressure and plasma power. At powers of less than 400 watts and pressures of a few atmospheres the vast bulk of the plasma is contained within the launching structure, so that given the opaque nature of the known surfatron devices very little of the light produced by the plasma can be harvested.
A typical surfatron structure is shown in diagrammatically in FIG. 1. The surfatron 1 has an HF structure consisting of two metal cylinders 2,3 forming a section of coaxial transmission line 4 terminated by a short circuit 5 at one end and by a circular gap 6 at the other. A HF electric field extending through the gap can excite an azimuthally symmetric surface wave to sustain a plasma column 7 of excitable material in a dielectric tube 8 arranged co-axially within the cylinders. A coaxial, cylindrical, capacitative coupler 9 is positioned between the cylinders, with a connection 10 extending out through outer cylinder. There it is connected to an input transmission line. A plate is attached to the inner conductor to form a capacitance between this plate and the inner metal cylinder.